A laser radar applies a laser beam onto a target (atmospheric air, fine grains or aerosols in the atmospheric air, constructs, or the like) from a remote place and receives a reflected light (scattered light) from the target, thereby making it possible to measure diverse information such as fine grains in the atmospheric air, the distribution of molecules, the velocity of wind, or a distance to a construction, from the remote place.
In most cases, the laser radar is employed outdoors, and requires a high reliability. Also, in order to measure information that pertains to a remote place with high precision, there is required a large output and a beam quality of diffraction limit. Further, a coherent laser radar that detects the velocity of wind or a velocity of an object to be measured requires a single frequency for a transmitted light because of the detection of a slight Doppler frequency shift of a scattered light, as well as requires a long pulse width to obtain a high velocity resolution. In addition, in the event of using the laser radar outdoors, the safety of human eyes is required. A laser beam having a wavelength of 1.4 μm or longer is called “eye safe wavelength” which is large in the tolerance for the eyes. Therefore, the use of a laser beam having a wavelength of 1.4 μm or longer makes it possible to achieve both of the large output and the safety of the eyes.
For example, in a conventional spatial resonator type coherent laser radar device that is disclosed in Optics Letters, Vol. 26, No. 16, p 1262 to 1264 (2001), there is employed a space type laser radar resonator in which a laser resonator is disposed in a space by using an Er, Yb: Glass laser medium that oscillates at a wavelength of 1.54 μm and an optical part such as a mirror.
Moreover, to achieve the unification of frequencies, there is used an injection seeding method in which a local light is injected into the resonator, and a laser oscillation is generated at the same frequency as that of the local light, and a resonator length control for conforming a longitudinal mode of the laser resonator to the wavelength of the local light.
In addition, to capture a longer pulse width, a space type laser resonator having a longer resonator length (2 m) has been used.
However, a laser medium that oscillates at a wavelength of 1.4 μm or longer has difficulty in generating a laser beam with a high efficiency because of a small gain in general. Also, in the space type laser resonator, the optical parts such as a mirror requires a high precision in the installation, and in the laser radar device that is used outdoors in most cases, the parts are readily out of alignment which arises from a change in temperature, vibrations, or an impact, thereby increasing a loss within the laser resonator. As a result, there arises such a problem that the output of a transmitted light varies. An adverse affection of the parts being out of alignment generally becomes more prominent as the gain of the laser medium is smaller or as the resonator is longer.
In addition, because it is necessary to control the resonator length in order to obtain the transmitted light of a single frequency, a problem occurs in that the single frequency output is not captured while the control is unstable. Moreover, because the resonator length control is realized by mechanically moving the optical parts within the resonator, there is a problem in that output deterioration arises from the optical parts being out of alignment during the operation. For that reason, the conventional spatial resonator type coherent laser radar device has difficulty in gaining a reputation for high reliability.
To solve a means for solving the above problems, a conventional optical fiber type coherent laser radar device using an optical fiber amplifier for a transmitted light source is disclosed, for example, in FIG. 8 in Proceedings (p. 144 to 146) of 11th Coherent Laser Radar Conference (Malvern, Worcestershire, UK, July 2001).
The optical fiber type coherent laser radar device shown in FIG. 8 includes a laser source 1 that outputs a laser beam that oscillates at a single wavelength to an optical fiber, a first optical coupler 2 of the optical fiber type, an optical modulator 3, an optical fiber amplifier 4, a transmitting/receiving light splitting device 5, a transmitting/receiving optical system 6, a second coupler 7 of the optical fiber type, an optical receiver 8, a signal processor 9, a first polarization controller 10, and a second polarization controller 11.
Here, the transmitting/receiving light splitting device 5 includes a first coupling optical system 21, an optical polarizer 22, a quarter wave plate 23, and a second coupling optical system 24 as shown in FIG. 9.
In the optical fiber type coherent laser radar device shown in FIG. 8, optical parts that pass from the laser source 1 to the transmitting/receiving light splitting device 5 via the optical modulator 3, optical parts that pass from the first optical coupler 2 to the second optical coupler 7 through the second polarization controller 11, and optical parts that pass from the transmitting/receiving light splitting device 5 to the second optical coupler 7 are formed of inline fiber type optical parts, and coupled with each other by means of single mode optical fibers (SMF), respectively.
Next, a description will be given of the operation of the conventional optical fiber type coherent laser radar device. A laser beam from the laser source 1 that oscillates at a signal wavelength (frequency f0) is branched into two by the first optical coupler 2, one of which is used for the local light, and the other of which passes through the first optical coupler 2 and the first polarization controller 10 as a transmitted light, and is then modulated by the optical modulator 3.
Here, the optical modulator 3 is formed of an acousto-optic (AO) element which is driven by pulses, shifts an optical frequency of the laser beam by a frequency f1, modulates a laser beam in the form of pulses, and outputs the laser beam. The pulsed laser beam is applied toward a target by the transmitting/receiving optical system 6 through the transmitting/receiving light splitting device 5 after being amplified by the optical fiber amplifier 4.
The transmitted light that has been applied onto the target is scattered into a received light upon receiving a Doppler shift (Doppler frequency fd) corresponding to a velocity of the target. The received light is split from the transmitted light in the transmitting/receiving light splitting device 5 through the transmitting/receiving optical system 6, and is then coupled with the local light in the second optical coupler 7.
A mixed light of the received light with the local light is detected in heterodyne by the optical receiver 8, and a beat signal having a frequency of a frequency difference (f1+fd) between the local light and the received light is outputted from the optical receiver 8. The beat signal is processed in the signal processor 9, and physical information such as a distance to a target, a velocity, a density distribution, or a velocity distribution is measured from the receive intensity of the received light, a round trip time, and a Doppler frequency.
In the transmitting/receiving light splitting device 5, the transmitted light and the received light are split by means of polarization. As shown in FIG. 9, a pulsed laser beam from the optical fiber amplifier 4 is used for the transmitted light, and enters the optical polarizer 22 as a substantially collimated beam by the first coupling optical system 21. The light polarizer 22 is so set as to transmit polarized components that are in parallel with a paper face and reflect polarized components that are perpendicular to the paper face. The transmitted light that has been reflected by the polarizer 22 becomes a linearly polarized light that is perpendicular to the paper face.
Then, the linearly polarized light is transmitted to the transmitting/receiving optical system 6 after being converted into a circularly polarized light by passing through the quarter wave plate 23. Assuming that there is no change in polarization attributable to the scattering of the target, the received light from the transmitting/receiving optical system 6 is a circularly polarized light and is transmitted through the quarter wave plate 23 so as to be converted into a linearly polarized light that is in parallel with the paper face. The received light that has been converted into the linearly polarized light is transmitted through the polarizer 22 and is coupled with an optical fiber that reaches the second optical coupler 7 via the second coupling optical system 24.
To minimize the transmission loss of the transmitted light in the transmitting/receiving light splitting device 5, it is necessary that the transmitted light from the optical fiber amplifier 4 is converted into the linearly polarized light perpendicular to the paper face. To achieve this, an adjustment is made by the first polarization controller 10 so that the transmitted light from the optical fiber amplifier 4 is converted into the linearly polarized light perpendicular to the paper face.
Also, in the optical heterodyne detection, to maximize the detection efficiency, it is necessary to conform the polarized face of the local light to that of the received light. For that reason, an adjustment is made by the second polarization controller 11 so that the polarization face of the local light substantially conforms to the polarization face of the received light.
As described above, in the conventional optical fiber type coherent laser radar device, all of the components except the transmitting/receiving light splitting device 5 and the transmitting/receiving optical system 6 include optical fibers. Thus, the conventional optical fiber type coherent laser radar device is strong in a change in temperature, vibrations, and impact and is high in reliability.
The conventional optical fiber type coherent laser radar device shown in FIG. 8 implements the optical fiber amplifier 4 using a single mode optical fiber for the high output of the transmitted light. In the single mode optical fiber, a light is propagated in a small core of several μm to ten and several μm in diameter. When an output of the transmitted light increases, a power density within the core becomes large, which originates a nonlinear effect such as Brillouin scattering or Raman scattering. Also, damage occurs in an interior of an optical fiber, an end surface of the optical fiber, an optical part within an inline type optical part, or the like. Under the circumstances, an output power of a transmitted light is restricted.
The present invention has been made to solve the above problems, and therefore it is an object of the present invention to provide a coherent laser radar device that realizes a high reliability and a high output of a transmitted light.